Electrical resistance furnace



Patented Feb. 4, 1930 UNITE@ STATES Partnr orrrca BROWFL BOVER AND CIE.,

F BDEN, SWITZERLAND ELECTRICAL RESISTANCE FURNACE Application lec'l September 9,192?, `Serial No. 222,640, and in Germany September 39, i926.

This invention relates to electric resistance furnaces, particularly those intended for annealing or other heat treatment of various materials.

The general object of the invention is the provision of a furnace for the purpose specifled which is adapted to complete the heat treatment within the shortest possible time. A further object is the provision of a furnace in which the charge may be rapidly broughtup to the desired temperature and maintained thereat without causing violet fluctuations in the current taken from 'the mains.

Another object is the provision of a furnace having the characteristics specied which can be automatically operated with the aid of comparatively simple and inexpensive switching apparatus.

Other and further objects will be pointed out or indicated hereafter, or will be obvious to one skilled in the art upon an understanding of the invention or its employment in practice.

ln the drawing forming part of this specitication l illustrate diagrammatically one arrangement embodying the invention, but it must be understood that this is presented for purpose of illustration only and is not to be accorded any interpretation calculated to limit the claims short of the true and most comprehensive scope of the invention in the art.

ln the drawing Figure 1 is a diagrammatic'representation of a furnace and control gear embodying the invention;

Figure 2a is a curve diagram showing the variations of charge temperature with time when operating the furnace in accordance with the invention;

Figure 2b is a curve diagram showing` the power consumption during the aforesaid operation;

Figure 3a is a curve diagram showing the variations of charge temperature with time when operating the furnace in accordance with prior art practice;

Figure 3b is the corresponding power consumption diagram.

ln operating electric heat treatment furnaces of the resistance type, the amount of power supplied is governed jointly by the maximum temperature which the surface of the charge maybe permitted to attain and by the duration of the treatment. The rst of these factors limits the amount of power which may be supplied in accordance with the temperature within the furnace. To obtain economical working, however, the time required for the. treatment must he reduced to a minimum andl this necessitates a large power input.

ln order to comply as far as possible with both these conflicting demands, it has hitherto been the practice to supply the power intermittently or, in other words, to interrupt the supply at definite intervals. According to this method energy is supplied at a rate such as will give a short heating up period, after which the supply is periodically interrupted, the frequency and length of the interruptions being chosen such that the charge does not exceed the maximum permissible temperature. The above method of operation presents several serious drawbacks,

however, not the least being the heavy strain on the automatic switches required to connect and disconnect the full power. Moreover, with large furnaces the switch gear required for this purpose is both complicated and costly, and attempts to lighten the duty of the switches by reducing the frequency of operation leads to correspondingly increased periods of interruption with the result that the difference between the minimum and maximum temperatures becomes very large. Finally the connection and disconnection of large amounts of power causes continual violent fluctuations in the load on the system with resulting disturbing influences on other services, such as lighting, and even on the supply station itself.

These disadvantages may be to a large eX- tent overcome by arranging the switch gear so that after the first disconnection of the power, on the charge attaining the maximum temperature, the subsequent reconnections are not made with full power but with a fraction of the same, this fraction being slightly more than is requiredto cover the radiation lOl) losses of the furnace. Thus a considerable time will elapse after switching on the reduced power before the maximum temperature is again reached and it becomes necessary to disconnectl Athe supply once more. The arrangement therefore enables the temperature of the furnace tobemaintained within the given limits with far less frequent switching operations than heretofore and with smaller currents, thus substantially in'- creasing the life of the switch apparatus and giving a better load on the system. In addition it is possible to obtain closer temperature control without an excessive number of switching operations.

This resultis obtained by the use of one or more single or two-way circuit breakers provided with automatic operating gear, and the invention contemplates a method of operation which comprises the automatic reduction of the power supply, when the maximum teinperature is reached, to a value equal to or slightly greater than the power lost by radiation, etc., from the furnace. In the arrangement provided the necessary switch gear is located on the furnace side of the main circuit breaker.

An understanding of the invention will be most quickly obtained by reference to Figure l of the drawing accompanying this specification, which illustrates in diagrammatic form an example embodying the invention. Let the reference numeral l designate an electrically operated circuit breaker controlled by the electrical contact-making thermometer 4. The circuit breaker l controls the resistances 3 of the furnace which may be changed from delta to star connection by the switch 2, the change being effected automatically through the medium of a no-load coil. The numeral 5 denotes a thermo-couple forming part of the thermometer 4.

The procedure during a heat is as follows Aftcr placing a fresh charge in the furnace the main circuit breaker l is closed and the switch 2 turned to the position corresponding to delta connection of the resistances and maintained in this position by the no-load coil. The furnace being under full power the charge is rapidly brought up to the maximum temperature. As soon as this is reached the apparatus 4 and 5 comes into action and the main circuit breaker is opened. vThe no-load coil of the switch 2 is thus de-energized with the result that the resistances 3 are automatically changed over to star connection, so that when the main circuit breaker is reclosed only one third of the full power is applied to the furnace, the noload coil of the switch 2 being short-circuited.

Figures 22L and 2b of the drawing show thel variation of temperature and power input respectively with time during the progress of a heat.

For purpose of comparison two similar curves have been drawn to illustrate the conditions obtaining when the full power is connected and disconnected each time (Figures 3a and 3b). It will be immediately evident that the new method gives smaller currents to make and break and also less frequent switching operations, thus ensuring longer life to the switch gear and preventing Violent fluctuations of load 011 the system.

trolling the power input to said resistances, v

means responsive to the temperature in the furnace for controlling the operation of said circuit, switching means controlling thein terconnection of the furnace resistances, and means responsive to the opening of the circuit breaker for effecting .the operation of said switching means comprising a no-voltage coil connected to said resistances and adapted to be supplied with power from said resistances when the resistances are connected in delta by said switching means, but short circuited if the resistances are connected in star.

-2. An arrangement for operating electric annealing furnaces, in which after the first interruption of current by the main switch the connection of the furnace resistances is changed in such a way that after the re-connection of the main switch the furnace no longer operates on the same power, characterized by the feature that in the leads to the furnace resistances beyond the main switch one or more disconnecting or changeover switches are arranged which by means of zero voltage magnets in consequence of the failure of the voltage automatically effect the connection of the furnaceresistances.

3. An arrangement as claimed `in, claim 1 characterized by the feature that the main switch for connecting and disconnecting the furnace resistances is controlled by a thermo element in the furnace associated with a temperature measuring instrument having contacts for maximum and minimum temperature, so that when the maximum temperature is reached for the first time, the main switch is automatically opened and a switch controlled by the no-voltage coil changes the connections of the furnace resistances and when the minimum temperature is reached the main switch is automatically closed on the changed resistance connections.

In testimony whereof I have signed my name to this specification.

PAUL SCHLENKER. 

